A girl who looks like she'll fulfill every sordid whim.
Used as a very simple scale:
Fuck Monkey or Sex Pixie.
Fuck Monkey: Usually in their late teens or early twenties, F.A.B. (firm and bendy).
Sex Pixie: As above, but not as obvious in their intent.

Oi! lads check out the fuck monkey at the bar. I bet she'll go around the world.